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Powering Aircraft of the Future From the Ground, With Lasers

It sounds like a great concept for a new toy: a laser-powered paper airplane.

But scientists at the Tokyo Institute of Technology have bigger ideas in mind, like tiny, unpiloted airplanes powered by laser beams from the ground that can endlessly observe volcanoes or monitor climate. In the longer term, they say, laser beams from satellites or high-altitude balloons may perhaps propel planes at several times the speed of sound at high altitudes where the air is too thin for jet engines to operate.

In the meantime, they have a laser-powered paper airplane.

In the June 10 issue of Applied Physics Letters, the scientists describe their folded plane, with a wingspan of about two inches and a weight of less than a hundredth of an ounce. At the back of the planes, they attached small aluminum targets to bounce the laser light.

Reflected particles of light impart a small force, like a stream of water from a garden hose hitting a beach ball. But that force is too small to lift even something as slight as their craft from the ground.

They then placed a droplet of water on the aluminum. Now, a pulse of laser light, in addition to bouncing off the aluminum, heated some of the aluminum into a superhot gas, which blew away the water like exhaust from a rocket engine.

''It's very hard to measure the water speed because it's very fast,'' said Dr. Takashi Yabe, a professor of mechanical engineering at the Tokyo institute and the lead author of the Applied Physics Letters article. The spray of water particles moved at speeds of at least 200 miles an hour, he said.

With the burst of thrust, the paper airplane lifted off from the laboratory workbench and glided to the ground.

''The significance of our idea is using water,'' Dr. Yabe said.

The laser power approach is appealing because the power source -- the laser -- is on the ground, saving weight on the aircraft.

A similar approach could also provide a cheap way to launch small satellites, an idea first proposed by Dr. Arthur R. Kantrowitz, now a professor of engineering at Dartmouth.

In October 2000, a carbon dioxide laser at White Sands Missile Range in New Mexico pushed a shiny, acorn-shape craft about the size of a softball 233 feet into the air.

That remains the record altitude for a laser-driven rocket. The flight lasted less than 13 seconds.

Dr. Leik N. Myrabo, a professor of mechanical engineering at Rensselaer Polytechnic Institute in Troy, N.Y., and designer of the laser-driven ''lightcraft,'' foresees going much higher in the future. A laser 10 to 100 times as powerful could be strong enough to accelerate a small craft to a speed six times as great as the speed of sound to the edge of space, he said.

''At least that's what the computer programs indicate,'' Dr. Myrabo said. ''This could completely change everything as far as the access to space.'' He is also chief executive officer of Lightcraft Technologies of Bennington, Vt., a company he founded to develop rockets driven by lasers and microwaves.

In the 2000 version of the spacecraft, a parabolic mirror on the bottom of the craft focused the laser pulses onto a layer of polymer, vaporizing it and providing the upward thrust. The engine is designed so that if wind starts pushing the craft out of line with the laser, thrust automatically adjusts to push it back.

Ultimately, something more akin to a usual jet engine may be more efficient, using the laser energy instead of burning jet fuel to heat air as it rushes through. ''That makes sense until you run out of air,'' Dr. Myrabo said.

At higher altitudes, water -- as in the Tokyo paper airplanes -- or liquid hydrogen could be the propellant.

''The potential advantages are that one could put payloads into orbit for pretty much the cost of the energy,'' said John W. Cole, manager of the revolutionary propulsion research project office at NASA's Marshall Space Flight Center.

The energy cost would be a few dollars per kilogram, Mr. Cole said, compared with the cost of current chemical rockets, $10,000 per kilogram. Even accounting for the energy absorbed by air molecules and other inefficiencies, the technology has the potential of cutting launching costs by a factor of 100 to 1,000, Mr. Cole and Dr. Myrabo both said.

But Mr. Cole added: ''It is not anywhere near mature. We do not have lasers big enough to actually implement this.''

The laser pulses also need to be much shorter to transfer energy to the rocket efficiently.

Closer to reality, scientists are investigating the use of lasers to push around satellites already in space. Dr. Andrew V. Pakhomov, a professor of physics at the University of Alabama at Huntsville, who is organizing a conference on beamed propulsion systems to be held in November, said bouncing laser beams could serve as tiny thrusters to adjust the position and orientation of satellites with much finer precision than current chemical thrusters. ''That's practically almost now,'' he said.

Lasers could also be used to provide propulsion to tiny microsatellites, too small for conventional rocket thrusters.

Dr. Yabe and his colleagues are among the scientists who have taken the original idea of beamed energy and found new potential uses. ''It opens up something you haven't thought about, maybe,'' Dr. Pakhomov said.

In their paper, the Tokyo scientists also described a second paper airplane that used an aluminum target coated with plastic, providing sufficient thrust to push the plane into the air. But water is the more intriguing fuel. The paper airplane received only a single push before the water propellant was used up, but Dr. Yabe speculated that a larger plane using this technology could continually replenish the water from the air. ''So we don't need to carry large amounts of water,'' he said.

For the next experiment, the researchers plan to use a laser to push a helium balloon. The laser pulses could also maneuver the aircraft by reshaping the wings while in flight.

One possible use for the laser-powered planes would be in tracking carbon dioxide and its role in global warming. The laser beam would provide propulsion, and by measuring how much of the beam was absorbed before bouncing back to the ground, would tell the concentration of carbon dioxide, a greenhouse gases blamed for warming temperatures.